Common utilities for jupyter-contrib projects. Includes:

• providing a notebook-4.2-compatible nbextension API in order to smooth over differences in versions 4.0 and 4.1
• common application components and cli scripts
• utility classes and functions for use in tests

• Get jupyter contrib app subcommands from pkg_resources entrypoints
• Add _maybe_copy and _should_copy to nbextensions private API, since the logger keyword arg was only added after notebook 4.1
• Also patch JUPYTER_RUNTIME_DIR environment variable in testing_utils.jupyter_env

Include almost-full 4.2 nbextensions public API

Move raise_on_bad_version to testing_utils package.

First public release!